Native calcium channels have been classified by their electrophysiological and pharmacological properties as T, L, N, P and Q types (for reviews see McCleskey, E. W. et al. Curr Topics Membr (1991) 39:295–326, and Dunlap, K. et al. Trends Neurosci (1995) 18:89–98). T-type (or low voltage-activated) channels describe a broad class of molecules that transiently activate at negative potentials and are highly sensitive to changes in resting potential. The L, N, P and Q-type channels activate at more positive potentials (high voltage activated) and display diverse kinetics and voltage-dependent properties. There is some overlap in biophysical properties of the high voltage-activated channels, consequently pharmacological profiles are useful to further distinguish them. L-type channels are sensitive to dihydropyridine agonists and antagonists, N-type channels are blocked by the Conus geographus peptide toxin, ω-conotoxin GVIA, and P-type channels are blocked by the peptide ω-agatoxin IVA from the venom of the funnel web spider, Agelenopsis aperta. A fourth type of high voltage-activated calcium channel (Q-type) has been described, although whether the Q- and P-type channels are distinct molecular entities is controversial (Sather, W. A. et al. Neuron (1995) 11:291–303; Stea, A. et al. Proc Natl Acad Sci USA (1994) 91:10576–10580; Bourinet, E. et al. Nature Neuroscience (1999) 2:407–415). Several types of calcium conductances do not fall neatly into any of the above categories and there is variability of properties even within a category suggesting that additional calcium channels subtypes remain to be classified.
Biochemical analyses show that neuronal high voltage activated calcium channels are heterooligomeric complexes consisting of three distinct subunits (α1, α2δ and β) (reviewed by De Waard, M. et al. Ion Channels (1997) vol. 4, Narahashi, T. ed. Plenum Press, NY). The α1 subunit is the major pore-forming subunit and contains the voltage sensor and binding sites for calcium channel antagonists. The mainly extracellular α2 is disulfide-linked to the transmembrane δ subunit and both are derived from the same gene and are proteolytically cleaved in vivo. The β subunit is a nonglycosylated, hydrophilic protein with a high affinity of binding to a cytoplasmic region of the α1 subunit. A fourth subunit, γ, is unique to L-type calcium channels expressed in skeletal muscle T-tubules. The isolation and characterization of γ-subunit-encoding cDNAs is described in U.S. Pat. No. 5,386,025 which is incorporated herein by reference.
Recently, each of these α1 subtypes has been cloned and expressed, thus permitting more extensive pharmacological studies. These channels have been designated α1A–α1I and α1S and correlated with the subtypes set forth above. α1A channels are of the P/Q type; α1B represents N; α1C, α′1D, α1F and α1S represent L; α1E represents a novel type of calcium conductance, and α1G–α1I represent members of the T-type family, reviewed in Stea, A. et al. in Handbook of Receptors and Channels (1994), North, R. A. ed. CRC Press; Perez-Reyes, et al. Nature (1998) 391:896–900; Cribbs, L. L. et al. Circulation Research (1998) 83:103–109; Lee, J. H. et al. Journal of Neuroscience (1999) 19:1912–1921.
Further details concerning the function of N-type channels, which are mainly localized to neurons, have been disclosed, for example, in U.S. Pat. No. 5,623,051, the disclosure of which is incorporated herein by reference. As described, N-type channels possess a site for binding syntaxin, a protein anchored in the presynaptic membrane. Blocking this interaction also blocks the presynaptic response to calcium influx. Thus, compounds that block the interaction between syntaxin and this binding site would be useful in neural protection and analgesia. Such compounds have the added advantage of enhanced specificity for presynaptic calcium channel effects.
U.S. Pat. No. 5,646,149 describes calcium channel antagonists of the formula A-Y-B wherein B contains a piperazine or piperidine ring directly linked to Y. An essential component of these molecules is represented by A, which must be an antioxidant; the piperazine or piperidine itself is said to be important. The exemplified compounds contain a benzhydril substituent, based on known calcium channel blockers (see below). U.S. Pat. No. 5,703,071 discloses compounds said to be useful in treating ischemic diseases. A mandatory portion of the molecule is a tropolone residue; among the substituents permitted are piperazine derivatives, including their benzhydril derivatives. U.S. Pat. No. 5,428,038 discloses compounds which are said to exert a neural protective and antiallergic effect. These compounds are coumarin derivatives which may include derivatives of piperazine and other six-membered heterocycles. A permitted substituent on the heterocycle is diphenylhydroxymethyl. Thus, approaches in the art for various indications which may involve calcium channel blocking activity have employed compounds which incidentally contain piperidine or piperazine moieties substituted with benzhydril but mandate additional substituents to maintain functionality.
Certain compounds containing both benzhydril moieties and piperidine or piperazine are known to be calcium channel antagonists and neuroleptic drugs. For example, Gould, R. J. et al. Proc Natl Acad Sci USA (1983) 80:5122–5125 describes antischizophrenic neuroleptic drugs such as lidoflazine, fluspirilene, pimozide, clopimozide, and penfluridol. It has also been shown that fluspirilene binds to sites on L-type calcium channels (King, V. K. et al. J Biol Chem (1989) 264:5633–5641) as well as blocking N-type calcium current (Grantham, C. J. et al. Brit J Pharmacol (1944) 111:483–488). In addition, Lomerizine, as developed by Kanebo KK, is a known calcium channel blocker; Lomerizine is, however, not specific for N-type channels. A review of publications concerning Lomerizine is found in Dooley, D., Current Opinion in CPNS Investigational Drugs (1999) 1:116–125.
The present invention is based on the recognition that the combination of a six-membered heterocyclic ring containing at least one nitrogen said nitrogen coupled through a linker to a benzhydril moiety results in effective calcium channel blocking activity. In some cases enhanced specificity for N-type channels, or decreased specificity for L-type channels is shown. The compounds are useful for treating stroke and pain and other calcium channel-associated disorders, as further described below. By focusing on these moieties, compounds useful in treating indications associated with calcium channel activity are prepared.